In the related art, a kind of steam turbine is known to include a plurality of stages each including a casing, a shaft body (a rotor) rotatably installed in the casing, turbine vanes fixedly disposed at an inner circumferential section of the casing, and turbine blades radially installed at the shaft body at a downstream side of the turbine vanes. In such a steam turbine, an impulse turbine converts pressure energy of steam into velocity energy by the turbine vanes, and converts the velocity energy into rotational energy (mechanical energy) by the turbine blades. In addition, in the steam turbine, a reaction turbine converts pressure energy into velocity energy also in the turbine blades, and converts the velocity energy into rotational energy (mechanical energy) by a reaction force applied by the steam burst.
In many cases of this kind of steam turbine, a gap in a radial direction is formed between tip sections of the turbine blades and the casing surrounding the turbine blades to form a flow path of the steam, and a gap in the radial direction is also formed between tip sections of the turbine vanes and the shaft body.
However, leaked steam passing through the gap of the turbine blade tip section toward a downstream side does not apply a rotational force to the turbine blades. In addition, since the leaked steam passing through the gap of the turbine vane tip section toward the downstream side does not convert the pressure energy into the velocity energy by the turbine vanes, the rotational force is hardly applied to the turbine blades of the downstream side. Accordingly, in order to improve performance of the steam turbine, it is important to reduce an amount of leaked steam passing through the gap.
Here, a structure shown in FIG. 9 has been proposed (for example, see Patent Literature 1). In this structure, for example, step sections 502 (502A, 502B, 502C) having heights gradually increased from an upstream side toward a downstream side in a rotary axis direction (hereinafter, simply referred to as an axial direction) are formed at a tip section 501 of a turbine blade 500. Seal fins 504 (504A, 504B, 504C) having micro gaps H101, H102 and H103 corresponding to the step sections 502 (502A, 502B, 502C) are formed at a casing 503.
According to the above-mentioned configuration, as a leakage flow passing through the micro gap H101, H102 and H103 of the seal fins 504 (504A, 504B, 504C) collides with end edge sections (edge sections) 505 (505A, 505B, 505C) forming step surfaces 506 (506A, 506B, 506C) of the step sections 502 (502A, 502B, 502C), a flow resistance can be increased. In addition, steam separated by the end edge sections 505 (505A, 505B, 505C) of the step surfaces 506 (506A, 506B, 506C) becomes a separation vortex Y100. The separation vortex Y100 generates a downflow from tips of the seal fins 504 (504A, 504B, 504C) toward the tip section 501 of the turbine blade 500. The downflow exhibits a contraction flow effect of the steam passing through the micro gap H101, H102 and H103. For this reason, a flow rate of the leaked steam passing through the micro gaps H101, H102 and H103 between the casing 503 and the tip section 501 of the turbine blade 500 is reduced.